Steel sheets overlapped on each other are typically joined by resistance spot welding which is one type of lap resistance welding.
Resistance spot welding is a method of squeezing two or more overlapping steel sheets by a pair of electrodes from above and below and, while applying an electrode force, passing a high welding current between the upper and lower electrodes for a short time to join the steel sheets. Heat generated from the resistance to the flow of the high welding current is used to obtain a spot weld. The spot weld is called a nugget, and results from the overlapping steel sheets melting and solidifying at their contact portion when the current flows through the steel sheets. The steel sheets are spot-joined by this nugget.
The joining strength of the resistance spot weld depends on the nugget diameter. Particularly in the case where high joining strength is required as in automotive parts and the like, it is important to ensure at least a predetermined nugget diameter.
Typically, in the case where the electrode force and the welding time are constant, the nugget diameter gradually increases as the welding current increases. When the welding current reaches a certain value or more, however, splashing occurs. Splashing is a phenomenon in which molten metal splatters between the steel sheets. Splashing not only is dangerous, but also degrades appearance as molten metal adheres around the weld, and causes variations in nugget diameter and joint tensile strength. This results in unstable joint quality.
Regarding the structures of automotive parts, for example, a center pillar has a structure in which a reinforcement is sandwiched between an outer portion and an inner portion. In this structure, three or more steel sheets need to be overlapped and spot welded, unlike in the case of simply spot welding two overlapping steel sheets.
Recent demand for more improved crash safety of automobiles has encouraged increases in strength and thickness of reinforcements and the like. This often creates the need to spot weld a sheet combination in which an outer portion (thin sheet) with small sheet thickness is located on the outer side and an inner portion and a reinforcement (thick sheets) with large sheet thickness are located on the inner side. Of the steel sheets in the sheet combination, a steel sheet with relatively small sheet thickness is referred to as a thin sheet, and a steel sheet with relatively large sheet thickness as a thick sheet. The same applies hereafter.
It is known that, in the case where such a sheet combination of three or more sheets with a high sheet thickness ratio ((the total thickness of the sheet combination)/(the sheet thickness of the thinnest steel sheet in the sheet combination)) is subjected to conventional spot welding of maintaining a constant electrode force and welding current, a nugget of a required size is hard to be formed between the outermost thin sheet (in contact with the electrode tip) and the thick sheet. This tendency is particularly noticeable when the sheet thickness ratio of the sheet combination is more than 3, or further 5 or more.
This is because the temperature tends not to increase between the outermost thin sheet and the thick sheet due to cooling by the electrode tip.
The nugget is typically formed by heat generated by volume resistance according to the specific resistance of each steel sheet from near the center between the electrodes. Since the nugget grows large between the thick sheets located near the center between the electrodes before the nugget grows to the thin sheet side, splashing occurs as it cannot be prevented by the applied electrode force. It is therefore difficult to obtain a nugget of a required size between the thin sheet and the thick sheet without splashing in such a sheet combination.
In the case where the outermost thin sheet is the outer portion, mild steel is often used as the steel sheet because formability is more important than strength. Meanwhile, the thick sheet is a strength reinforcing member, for which a high tensile strength steel sheet is often used. In such a sheet combination, the heat generation position tends to be closer to the high tensile strength steel sheet with high specific resistance. This further hinders the nugget formation between the thick sheet and the thin sheet (mild steel). Besides, when the steel sheet used is a coated steel sheet, the coated layer that has molten at a low temperature expands the current path between the steel sheets, causing a decrease in current density. This makes the nugget formation on the thin sheet side more difficult.
Thus, a nugget of a required size is hard to be formed between the thin and thick sheets in the aforementioned sheet combination of three or more sheets with a high sheet thickness ratio. The welding condition for obtaining an appropriate nugget diameter is therefore very limited.
In addition, in the case where disturbances are present during welding such as when a point that has already been welded (existing weld) is present near the current welding point or when the parts to be welded have significant surface roughness and a contact point of the parts to be welded is present near the welding point, part of the welding current is shunted into such an existing weld or contact point during welding. In this state, even when welding is performed under a predetermined condition, the current density at the position to be welded which is directly above or below the electrodes decreases, and so a nugget of a required diameter cannot be obtained. To compensate for such an insufficient amount of heat generated and obtain a nugget of a required diameter, a high welding current needs to be set beforehand.
Moreover, in the case where the surroundings of the welding point are firmly restrained due to surface roughness, member shape, etc., a larger sheet gap between the steel sheets causes a smaller contact diameter of the steel sheets, which may hinder the obtainment of a nugget of a required diameter or facilitate splashing.
Given that the welding condition for obtaining an appropriate nugget diameter in a sheet combination of three or more sheets with a high sheet thickness ratio is very limited as mentioned above, these disturbances may have significant effects.
As a resistance spot welding method for such a sheet combination of three or more sheets with a high sheet thickness ratio, for example, JP 2003-071569 A (PTL 1) proposes the following technique. In a sheet combination with a high sheet thickness ratio in which a thin sheet is further overlapped on two overlapping thick sheets, a bearing surface one level higher than a general portion is partly formed at the welding position of the thin sheet, and the end of the electrode facing the thin sheet is made spherical. In initial welding, the thin sheet and the adjacent thick sheet are welded with a low electrode force so as to crush the bearing surface of the thin sheet. After this, the two thick sheets are welded with a high electrode force. A required nugget is thus formed between the thin and thick sheets.
JP 2003-251469 A (PTL 2) proposes the following technique. In a method of squeezing, by a pair of electrode tips, a workpiece in which a thin sheet with low rigidity is overlapped on two thick sheets with high rigidity and spot welding the workpiece, the electrode force applied to the workpiece by the electrode tip in contact with the thin sheet lowest in rigidity is lower than the electrode force applied to the workpiece by the electrode tip in contact with the thick sheet, to form a nugget between the thin and thick sheets. The weld strength of the workpiece is thus enhanced.
JP 2004-358500 A (PTL 3) proposes the following technique. In a method of spot welding parts to be welded with a high sheet thickness ratio, after passing a welding current while applying a first electrode force to the parts to be welded, the current passage is stopped, and then a welding current is passed again while applying a second electrode force higher than the first electrode force in the state where the parts to be welded remain squeezed. Desirably, the current value of the welding current in the process of applying the first electrode force is changed in three steps of first to third steps, where the current in the second step is less than the current in each of the first and third steps. The joining strength of the parts to be welded with a high sheet thickness ratio is thus improved.